1. Field of the Invention
The present invention relates to a method for generating mixed light colors from an individually pulse-time controllable energization of light sources for colors whose brightnesses can be influenced by varying periodically successive duty ratios of the current flowing via the respective light source.
2. Discussion of the Prior Art
Measures of this type are known from DE 10 2004 047 669 A1 (in particular in connection with FIG. 3a and FIG. 4b therein). According to this document, light sources of the three primary valences (primary colors) red, green and blue are operated periodically with a constant current with duty ratios which can be set independently of one another, and their color emissions are additively mixed. Light sources such as lasers, electroluminescence elements, organic LEDs or in particular semiconductor light-emitting diodes are preferably used since their brightnesses are approximately linearly dependent on the duty ratio of the feeding with the pulse-time-modulated constant current pulses. The resultant mixed light color locus can be represented in the CIE standard chromaticity diagram depicted schematically therein (FIG. 6). This color locus can accordingly be displaced via at least one of the three primary-colored brightness contributions. Thus, each mixed light color can be set within a color triangle which is inscribed in the standard chromaticity diagram and whose corner points are given by the individual color emissions of the three primary-colored light sources used for the mixed light illumination.
Via the individual duty ratios, the respective intensity of the contribution of the primary colors to the mixed light color impression can be varied and, as a result, the color locus can be altered in a targeted manner. The shorter the periodically recurring switch-on time span, then the shorter the respective constant current pulse and thus the smaller the current-time integral of the current flow via the respective LED and accordingly the lower the brightness of the single-colored light contributed by the latter. With such brightness dynamic characteristics by way of the pulse time control, however, it is usually only possible to obtain a dimming ratio of the order of magnitude of 1:1000 between dark and bright. This no longer suffices e.g. for changing over color-variable twilight impressions in the case of extremely low brightnesses of the LEDs. Moreover particularly when, given an already high degree of dimming, small color locus corrections are additionally necessary for compensation of current-flow-dependent color locus shifts, that is to say for the so-called gamut color corrections, it should be endeavored to achieve a dimming ratio that is higher by at least one order of magnitude, that is to say an even weaker driving before the complete switching-off of the LEDs.
Precisely this gamut color correction required for high-quality, color-constant illumination effects necessitates very short current flow times via the light-emitting diodes. It is thus possible to compensate for example for the fact that the color loci of the LEDs vary owing to production. In order nevertheless to be able to represent a predetermined primary color, already during production adjustment or later during operation, the other two primary colors are admixed at extremely low intensities such as arise for the respective color locus from the CIE standard chromaticity diagram. By way of example, a guaranteed color locus “blue, unsaturated” is generated in gamut-corrected fashion by virtue of the fact that, in addition to the full driving (100%) of the blue LED, the green LED is driven at 5% and the red LED is driven at 2%. In order to represent this color locus at low brightness, for instance dimmed to 1%, in a driving period of 3 ms duration, for blue a switch-on time of 1% of the total period, that is to say 30 μs, arises, for green 1% of 5% equal to 0.05% (1.5 μs) and for red 1% of 2% equal to 0.02% (0.6 μs current flow via the red LED). Such intensive LED dimming by means of extremely short current pulses can only be realized with very fast and therefore expensive processors owing to the high coding depth required for such finely graded quantization, together with powerful high-frequency transistors as constant current sinks for the LEDs; that is to say with rarely tenable outlay on circuitry.
In order to avoid peak loads, for instance on an on-board power supply system with isolated operation, the LEDs of the three primary colors are not switched on simultaneously but rather in a manner temporally offset with respect to one another periodically in pulse-time-controlled fashion in order, on account of the integrating effect of the human eye, to produce the resultant mixed color impression. However, such differently colored pulse illuminations which are successive in different lengths, in particular if appropriate even without any mutual temporal overlaps, can physiologically be perceived as disturbing. This is because a discontinuous illumination results in a color separation effect that is disturbing to the human eye, with the result that—especially on an object moving in front of a background—no stable color locus appears under certain circumstances. In addition, the periodic colored light emissions lasting for different lengths can bring about irritating stroboscopic effects in particular on periodically moving objects which as a result are irradiated in intermittent fashion; and floating phenomena if objects are irradiated with frequencies that differ slightly from one another, such as, for instance, by light sources fed from unsynchronized power supply systems with isolated operation.